Patterning of a relatively thick copper layer or a thinner seed copper layer has been a requirement of printed circuit fabrication techniques. More recently, it has been desired to pattern thin copper layers in integrated circuit fabrication technology. Aluminum has been commonly used successfully in integrated circuit technology to form conductive leads between and across active elements of the integrated circuits, partly because aluminum has been found to be easily patterned by several techniques. However, as discussed by Hu, et al., in "Diffusion Barrier Studies for CU", 1986 IEEE V-MIC Conference, Jun. 9-10, 1986, pages 181-187, copper offers several advantages over aluminum metallization layers in integrated circuits such as higher conductivity, better electromigration resistance, and reduced power consumption. As further discussed, however, in the same article, copper has a greater tendency to diffuse into silicon than aluminum. Satisfactory solution to this problem has, nevertheless, been found by the use of diffusion barriers between the silicon and other layers of integrated circuits and the copper metallization layers.
Patterned etching of copper films has been accomplished, primarily in printed circuit fabrication, by wet processes wherein a protective, patterned film, such as a photoresist is applied to a copper layer and then a strong liquid etchant, usually an acid, is applied to etch the exposed copper down to the base material. This process presents several problems when applied to integrated circuit fabrication, such as the following. The wet processes are inherently "dirty" in that contaminants in the etchant can be introduced to the integrated circuit wafer. The wet etchants required are generally hazardous to operators by contact and inhalation of vapors produced thereby. Etching of copper requires etchants, high temperatures, or both which may damage the other layers of an integrated circuit. Etching of copper by wet processes is isotropic, making copper metallization for VLSI circuits extremely difficult. Disposal of the waste products of wet etch processes is becoming more expensive.
Therefore, because of the increased desire to utilize copper metallization in integrated circuits and the problems inherent in known wet etch techniques, it has become more important to develop more effective etching processes and equipment for etching copper layers, especially as used in integrated circuit fabrication.
Sputtering techniques are utilized presently to remove copper from copper-doped aluminum films and therefore may have application to removing copper films. However, sputtering, the physical dislodging of copper atoms and clusters by high energy ion bombardment, does not exhibit selective removal of various films and will result in extreme build-up of residue in the reaction chamber, since copper is not converted to a volatile species. Also, sputter removal techniques are relatively slow, difficult to control, and exhibit insufficient selectivity to masking layers.
Dry etch processes involving a plasma to form reactive agents such as a halogen, an amine, or an organic radical which react and form volatile copper products could be an approach to an effective etch process for copper which solves some of the problems encountered in other processes, particularly the redeposit of residue on reaction chambers. However, due to the known high melting and boiling points of the copper compounds that would be formed, coupled with the normal range of substrate temperatures associated with these plasma processes, it has been thought that volatilization would not occur and that these processes would be unsuccessful. Moreover, a plasma discharge produces a wide range of disassociated, reactive products that can combine homogenously or heterogeneously to form a polymer residue. For example, hydrocarbon and chlorocarbon can produce heavy residues of polymer generating many particles. Also, if the reaction product of these processes does not volatize, it has been thought that the surface reaction would prevent further reaction of the copper bulk material below this surface. Some investigation of the reaction of chlorine with copper has been done as discussed by W. Sesselmann, et al., in "The Interaction of Chlorine with Copper", Surface Science, vol. 176(1986) pages 32-90. Some early investigation of ion etching of copper films is discussed by Schwartz, et al, in "Reactive Ion Etching of Copper Films", Journal of the Electochemical Society, Vol. 130, No. 8 (1983), pages 1777-1779.